Today I'm walking along a feature near Nelson, New Zealand called the Boulder Bank.
And this boulder bank is like a spit - like you might see a sand spit - but it's made
of boulders as opposed to sand. And it's 13 kilometers long, even a bit longer than that,
and it extends in a really narrow ridge parallel to the coastline just to the north of Nelson City.
And this boulder bank is a bit of a mystery because its origins are not very easy to understand.
So the source area for all of these rocks - it seems at least at face value -
to be just further north at a place called Mackay Bluff. And that is a very distinct rock type,
it's called granodiorite, and it's the only place that this material actually outcrops in this area.
So it seems that that's the source area - it makes sense, it's along the coast. And so
the obvious explanation is that the waves have just gradually washed sideways from the
source area and just washed these boulders along to extend the boulder bank over time.
So that's one possible explanation. The problem is that some of these boulders are huge and even
at the far end of the boulder bank you can find down at low tide level some really big boulders
maybe two meters across so how did the sea wash two meter boulders 13 kilometers along
the beach ridge. So let's just have a little think about this and let's have a closer look
at what the beach ridge is actually made of. You can see that on the seaward side of the ridge
there are these linear storm ridges with wood all over them and these are typically made in
a high tide with a strong wind and the waves wash the material up into a heap
and they can build up over many many years. And certainly you can see also that some of the wood
has washed right over the top of the ridge so it kind of is fairly clear that the ridge itself is
a storm ridge that has been washed up by the waves in heavy weather over time.
Shells that have been dug up from within the ridge and below the ridge suggest that it's
only about 7 000 years old or less, which is not very old in geological terms.
So that's a very interesting fact. And the other thing is that when you go down to
the sea level and below there is actually a whole platform extending out from the ridge
made up of huge boulders, really big ones, maybe a meter across or more. And underneath
this upper layer of big boulders is a mixed layer of unconsolidated gravels boulders pebbles
all sort of mixed up. So it suggests that the source area for the ridge itself is actually the material
below the big boulders of the platform and the big boulders themselves are perhaps what's left behind
when the sea has kind of washed up the smaller material up onto the boulder ridge. So the second
explanation is that this ridge hasn't actually all immediately derived from up at Mackay Bluff
but that it's been washed up from an area extending a little bit out to sea, and this area
is in some places about a thousand metres wide. But that leaves us with another problem which is:
how did that platform of big boulders and unconsolidated material get there in the
first place in the last seven thousand years? and I think as far as I know that is still an unsolved
mystery. So could it even be that Mackay Bluff is not actually the source area for most of these
boulders because there is some other source of the rocks offshore of the boulder bank,
perhaps a linear feature that has been eroded away and is now under the waves? Who knows?
This amazing feature the Nelson Boulder Bank, it's a globally rare boulder bank - one of the longest
in the world I believe. And it's a great feature for walking along, it encloses mud flats which
are great for bird life. So it's quite a special feature and I wonder what you think? Were these
rocks washed all the way along in one go in the last seven thousand years or were they washed up
from some other feature that had been moved into place long before 7000 years ago. So there you
have it. I hope you like this little geological mystery that we have here in Nelson New Zealand.
Once again a very warm welcome aboard!
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